Namura et al., U.S. Pat. No. 5,473,018 discloses the addition of polytetrafluoroethylene having a heat of crystallization of at least 50 J/g to PFA to increase the surface smoothness of the PFA. '018 characterizes this polytetrafluoroethylene as being low in molecular weight in contrast to polytetrafluoroethylene molding powder that has a low heat of crystallization and a molecular weight of several million, enabling the latter to be molded compression pre-molded/sintered or paste extrusion/sintered. In addition to reporting surface smoothness, '018 reports the effect of the low molecular weight polytetrafluoroethylene on the flex life (folding endurance) of the resultant composition depending on the amount of this component in the PFA-containing composition as follows: increase in flex life up to 5 wt %, decline in flex life at 10 wt %, and sharp decline at 20 wt % (Table 3), sharp declines at 10 wt % (Table 4) and sharp declines at just 5 wt %.
Namura, in later patents, U.S. Pat. Nos. 6,649,669 and 7,030,191, addresses this shortcoming in flex life by certain requirements for the PFA component and the low molecular weight polytetrafluoroethylene component. '669 requires that the perfluoro(alkyl vinyl ether) content of the PFA be at least 4 wt %, preferably 5 to 20 wt %, and the melt flow rate (MFR) of the PFA be no greater than 4 g/10 min, and the amount of the low molecular weight polytetrafluoroethylene be 5 to 30 wt %. In Tables 1 and 2, the peak flex life is at 10 wt % of this component. '191 discloses further improvement when the MFR of the PFA is 0.1 to 1.7 g/10 min, the perfluoro(alkyl vinyl ether content of the PFA is 5 to 10 wt %, preferably 6 to 10 wt %, and the amount of low molecular weight polytetrafluoroethylene in the composition is 30 to 55 wt %. Table 1 discloses the profound effect of PFA MFR on flex life, viz. as the MFR increases from 1.6 to 11.9 g/10 min the flex life decreases by 95%.
Low MFR of the PFA means that the PFA has low fluidity (high melt viscosity) in the molten state required for melt fabrication than high MFR PFA. Low fluidity, low MFR PFA is more difficult to melt fabricate than high fluidity, high MFR PFA. The problem is how to improve the flex life of PFA without the limitation that the PFA must have a low MFR, no greater than 4 g/10 min ('669) or no greater than 1.7 g/10 min ('191).